The Alpha Stirling Engine Configuration

The alpha Stirling engine consists of two cylinders which have a 90 degree angle between their linear working paths. Both pistons are connected at the same point on the crankshaft and the two cylinders are connected by a pipe. As with all Stirling engines the gas called the working fluid never leaves these cylinders and pipes. There are no valves as in other engine types therefore no other gas e.g. exhaust fumes moves in or out of the engine. This working fluid is usually air, hydrogen or helium. The two pistons are joined in such a way that they're linear motion can be translated into rotational motion which can be used to power a mechanism like a power generator. On the outside of the cooling cylinder there could be cooling fins as in a normal air cooled combustion engine or a radiator system which would use liquid to cool the engine as in most modern cars. The heating cylinder would have an external heating source which could be a fuel burner e.g. gas or petroleum or it could be a renewable energy source like solar power. The alpha Stirling engine is similar to a two cylinder two stroke combustion engine in that each cylinder produces a power stroke in one rotation of the crankshaft.

Low coefficient of friction, non-lubricated materials are used in order to prevent the use of lubricants which can clog the regenerator or the heat exchangers. These can be made from materials such as Rulon or Graphite. Rulon is a plastic and good choice of material because it does not require lubrication, is highly corrosion resistant and will work perfectly between -240oC and 288oC. Parts are designed to have low normal forces so that they don’t place too much pressure on any bearings which might lead to fatigue or fracture.As with other heat engines the working fluid in the Stirling engine goes through four stages cooling, compression, heating and expansion. This is done in the alpha Stirling engine by moving the working fluid to the cooling cylinder and then through the regenerator to the heating cylinder. The change in temperature will cause the pressure of the working fluid to change and the movement of the pistons will also change the pressure.

This means that when the working fluid enters the heating cylinder it will be heated and expanded. As the working fluid expands it will firstly push the heating piston towards the bottom of its cylinder. This movement will push the cooling piston to the top of its cylinder. When the heating piston has reached half of its power stroke the cooling piston will begin its power stroke. Therefore at this point both pistons will be producing power. When the heating piston reaches the bottom of its cylinders it can go no further so the cooling piston will continue to provide power until it has reached the bottom of its cylinder. At this point the cooling piston is at the bottom of its cylinder and the heating piston is half way up its cylinder. The momentum created by the flywheel will spin the engine until the pistons return to they're original position and the heating piston can push again.

How the Alpha Stirling Engine works

This is a thermodynamic cycle which means it is a series of thermodynamic processes which will change the gas and then return it to its original state. The Stirling cycle is reversible which means that if mechanical power is applied to the machine it will have a heating or cooling effect. The Stirling cycle differs from others as the working gas never leaves the machine i.e. it is a closed cycle which means that the thermodynamic processes change the pressure of the working gas which is used to produce mechanical work.

Pressure – Volume (P-v) and Temperature – Entropy (T-s) diagrams are used to show an ideal cycle of the working fluid.

From phase two to phase three the working fluid is being heated this means that at this point the working fluid is in the heating cylinder it will also have passed through the regenerator which will have helped to raise the temperature. This is shown as a sharp rise in pressure on the P-v diagram as the working fluid expands with no increase in volume. This sharp rise is as a result of Gay-Lussac's law; the volume remained constant so a rise in temperature caused the pressure to do the same. At the end of this phase the regenerator is cold.

From phase three to phase four the temperature remains constant as the heating piston is still near the bottom of its cylinder. The pressure now falls as it pushes the cold piston down to the bottom of its cylinder and the volume of gas increases. Because the Stirling engine is a closed system when the volume increased the pressure had to decrease in accordance with Boyle's law.

From phase four to phase one the working fluid passes the regenerator and begins to cool down. It enters the cold cylinder and is cooled down further. This is shown as a sharp decrease in pressure as the gas contracts but the volume remains the same. Again as the volume was constant and temperature was decreasing the pressure had to decrease too because of Gay-Lussac's law.

From phase one to phase two the cold piston is at the bottom of its cylinder which causes the gas to continue to contract. The momentum of the flywheel push's the heating piston to the top of its cylinder and the cooling piston half way up its cylinder which decreases the volume of working fluid and slightly increases the pressure. The temperature is constant so as the volume decreases the pressure must rise because of Boyle's law.